Patients with metastatic breast cancer, that is biopsy proven or clinically documented by standard imaging (e.g. CT, MRI, Bone Scan, Ultrasound, FDG PET/CT), may be eligible for this study.  Patients may participate in this study, if they are at least 18 years of age.  We anticipate enrolling up to 30 participants with metastatic disease who meet eligibility requirements for this study.

Positron emission tomography (PET/CT) imaging will be used to evaluate sigma-2 (σ₂₎ receptor activity in sites of metastatic breast cancer using the investigational radiotracer [¹⁸F]ISO-1.  This is an observational study in that [¹⁸F]ISO-1 PET/CT will not be used to direct treatment decisions.  While patients and referring physicians will not be blinded to the [¹⁸F]ISO-1 PET/CT results, treatment decisions are made by the treating physicians based upon clinical criteria. 

Patients with biopsy proven or clinically documented metastatic disease, who are starting new systemic treatment, will be enrolled and the primary outcome is progression free survival.  Patients will undergo one [¹⁸F]ISO-1PET/CT scan at baseline prior to starting new therapy, and a second [¹⁸F]ISO-1 PET/CT scan at approximately 3-6 weeks after starting new therapy.  An Optional 3^rd [¹⁸F]ISO-1 PET/CT scan may occur at the time of clinical progression.

Patients may undergo metastatic biopsy as part of their clinical care.  Patients may also be enrolled in a separate breast cancer biopsy protocol entitled “Metastatic Markers of Recurrent Tumor Phenotype for Breast Cancer” (METAMORPH).  We will ask permission to access tissue collected as part of clinical biopsy or METAMORPH, if available.  If the patient gives permission, archived tissue may be accessed for the purposes of this study to perform additional experimental pathology assays.  If the subject is not part of the METAMORPH study and is not undergoing a clinical biopsy as part of their standard care, they may be asked to undergo an optional research biopsy for the purposes of this study, if they have a site of disease that is deemed appropriate for biopsy by a physician.  Subjects may still participate in this study even if they choose not to have this optional research biopsy.  Experimental pathology tests may be performed on this tissue, if available including but not limited to assays for Ki-67, TK1, and mitotic index.   Results of the standard breast cancer pathology immunohistochemistry (IHC) panel (usually estrogen receptor (ER), progesterone receptor (PgR) and HER2/neu) will be collected from the medical record for both metastatic breast cancer biopsy and primary breast cancer biopsy or surgery, when available.